Post-extra systolic potentiation (PESP) is a property of cardiac myocytes that results in enhanced mechanical function of the heart on the beats following an extra systolic stimulus delivered early after either an intrinsic or pacing-induced systole. The magnitude of the enhanced mechanical function is strongly dependent on the timing of the extra systole relative to the preceding intrinsic or paced systole. When correctly timed, an extra systolic stimulation pulse causes an electrical depolarization of the heart but the attendant mechanical contraction is absent or substantially weakened. The contractility of the subsequent cardiac cycles, referred to as the post-extra systolic beats, is increased as described in detail in commonly assigned U.S. Pat. No. 5,213,098 issued to Bennett et al., incorporated herein by reference in its entirety.
Generally, PESP therapy has been proposed in two forms, “paired pacing” and “coupled pacing”. In paired pacing, the primary systole is a paced event followed by a paced extra systole delivered in the same chamber as the paced event, at a selected extra systolic interval (ESI) following the primary pacing pulse. In coupled pacing, the primary systole is an intrinsic depolarization followed by an extra systolic pacing pulse delivered in the same chamber as the sensed intrinsic depolarization at a selected ESI following the primary depolarization.
As noted, the degree of mechanical augmentation on post-extra systolic beats depends strongly on the timing of the extra systole following a first depolarization. If the ESI is too long, the PESP effects are not achieved because a normal mechanical contraction takes place in response to the extra systolic stimulus. As the ESI is shortened, a maximal effect is reached when the ESI is slightly longer than the physiological refractory period. An electrical depolarization occurs without a mechanical contraction or with a substantially weakened contraction. When the ESI becomes too short, the stimulus falls within the absolute refractory period and no depolarization occurs. As such, methods for controlling the ESI and for delivering pacing pulses in a manner that achieves an optimal PESP effect are needed.